Host cell proteins (HCPs), proteins made by expression systems alongside the desired therapeutic proteins, are a headache for industry. If they are not removed from the finished drug they can impact potency and stability. They can also provoke an unwanted immune response in patients.

Protein drug and monoclonal antibody manufacturers have worked out how to identify, quantify, and minimize HCPs in finished products through years of effort and method development, says Daniel Bracewell, PhD, from University College London.

“Work on HCPs over the last decade has revealed their impact goes beyond potential immunogenicity concerns to that of interactions with the manufacturing process, “ Bracewell says.

For example, enzyme-linked immunosorbent assay (ELISA) and mass spec-based analytical methods have been used to show that a group of enzymatic HCPs (lipases) can degrade polysorbate, which is significant because the compound is used to stabilize mAbs.

The same analytical techniques have identified other problematic HCPs, such as phospholipase B-like 2 (PLBd2), that can have a negative impact on manufacturing and product stability, prompting industry to develop removal and control processes, notes Bracewell.

HCP knowhow

The efforts made by the therapeutic protein sector will benefit the wider industry, according to Bracewell, who argued in a recent paper that gene therapy developers are likely to encounter similar challenges with HCPs.

“As with the evolution of understanding of HCPs in mAbs there are likely interactive effects we have yet to discover for gene therapies,” he continues. “The additional complexity of viral vectors whereby they have internalized structures and potential encapsulation of HCPs increases the difficulty of describing how HCPs can make their way through a manufacturing process.”

In the study Bracewell and colleagues write that although gene therapy developers are trialling mass spectrometry for the analysis of HCPs in viral vector preparations the approach is yet to be widely applied into process or host cell line development to reduce HCP amounts or risk.

“As these MS approaches, and the data from them, are applied and become available, the process understanding created will speed process development” and “inform risk assessment and management based on a knowledge of specific HCPs, ultimately delivering safe and efficacious gene therapy products to the clinic,” he explains.


Another advantage, according to Bracewell, is that many of the technologies needed to analyze HCPs are already well established, citing ELISAs and mass spec as examples of technologies used in the mAb space that can be adopted by gene therapy developers.

There are exceptions. Gene therapy developers–indeed any companies that use viral vectors–will have to develop some specific methods for HCP analysis, according to Bracewell.

“One outstanding question for HCPs in the viral vector space is should we fractionate encapsulated HCPs to understand the challenges these pose as distinct from HCPs making their way into the product by other means? If so, can these methods be standardized,” asks Bracewell.

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